The technology of producing semiconductor devices has been continually pressured to increase effective device densities in order to remain cost competitive. As a result, Very Large Scale Integration (VLSI) and Ultra Large Scale Integration (ULSI) technologies have entered the sub-micron realm of structural dimension and now are approaching physical limits in the nanometer feature size range. In the foreseeable future, absolute atomic physical limits will be reached in the conventional two-dimensional approach to semiconductor device design. Traditionally, dynamic random access memory (DRAM) designers have faced the severest of challenges in advancing technologies. For example, designers of 64K DRAMs were perplexed to learn that a practical physical limit to charge capacity of storage capacitors had already been reached due to the minimum charge necessary to sense signals in the presence of environmental or particulate radiation inherently present in fabrication materials. Storage capacitors in the range of 50 femtofarads are now considered to be a physical limit. From a practical view, this limitation prevented the scaling of DRAM capacitors. Reduction of the surface area of a semiconductor substrate utilized by the storage capacitor has also been severely restricted. Due to decreases in the thickness of capacitor materials, existing 1 Megabit (1 MBit) DRAM technologies utilize a planar device in circuit design. Beginning with 4 MBit DRAMs, the world of three-dimensional design has been explored to the extent that the simple single device/capacitor memory cell has been altered to provide the capacitor in a vertical dimension. In such designs the capacitor has been formed in a trench in the surface of the semiconductor substrate. In yet denser designs, other forms of capacitor design are proposed, such as stacking the capacitor above the transfer device.
The progress of DRAM technology, which in many ways drives micro-electronics technology, is thus currently limited in significant part by the difficulty of fabricating storage capacitors with sufficient capacitance within decreasing area on the chip. The DRAM world is currently divided between two paths, with some manufacturers pursuing trench capacitors built into the crystalline silicon wafer, and other manufacturers pursuing stacked capacitors in which the capacitor is fabricated on top of the wafer surface. The use of a stacked capacitor permits a variety of new process options, for example, in the choice of electrode material (polysilicon, silicide, etc.). In the case of the trench capacitor, its extendibility is in doubt since it is extremely difficult to etch about 0.15-0.25 micrometer wide trenches well over 10 micrometers deep, as well as to then fabricate ultrathin dielectric layers on the trench surface, fill the trench, etc.
A need thus continues to exist in the art for a capacitor having a large surface area so that the capacitor's capacitance is increased, without increasing the area occupied by the capacitor structure on or in a silicon substrate.